A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter. The duty cycle “D” is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
A power converter with a low power rating designed to convert an alternating current (“ac”) mains voltage to a regulated dc output voltage to power an electronic load such as a printer, modem, or personal computer is generally referred to as an “ac power adapter” or a “power adapter,” or, herein succinctly, as an “adapter.” Industry standards have required continual reductions in no-load power supply loss to reduce power consumed by millions of power adapters that remain plugged in, but are not in use. Efficiency requirements at very low output power levels were established in view of the typical load presented by an electronic device in an idle or sleep mode, which is an operational state for a large fraction of the time for such devices in a home or office environment.
No-load power loss of a power adapter is typically dominated by three phenomena. The first phenomenon is directed to the current drawn from high-voltage supply bus to provide power to the controller of the adapter. The high-voltage power draw is sometimes shut off when the adapter is in operation and power can be supplied from an auxiliary winding of a transformer thereof. However, in the absence of operation of the adapter (e.g., a start-up condition or in the event of complete shutdown of the controller), the high-voltage supply bus provides power to the controller directly. While the current required to start or maintain controller operation may be small, the fact that it comes from a high-voltage bus causes a higher-than-optimal draw of power from the input of the adapter.
The second phenomenon is directed to the current flow in a bleeder resistor coupled across an “X-capacitor” (i.e., a safety rated capacitor) of the adapter. An X-capacitor is a capacitor coupled across the ac input power mains (also referred to as “ac mains”) to a power converter to reduce electromagnetic interference (“EMI”) produced by the power converter and conducted back to the ac mains. A “Y-capacitor” (i.e., a safety rated capacitor) is an EMI-reducing capacitor coupled between ac mains to a power converter and an input-side grounding conductor. Both the X-capacitor and the Y-capacitor are distinguished by a safety voltage rating related to a peak voltage that the respective capacitor is required to sustain. Upon disconnection from the ac mains, the X-capacitor should be bled down to a low voltage in a short period of time. Bleeding down an X-capacitor voltage is typically accomplished with a bleeder resistor coupled across the capacitor.
The third phenomenon is directed to gate drive and other continuing power losses that do not vary with load. The third phenomenon is commonly addressed by using a burst-mode of operation, wherein the controller is disabled for a period of time (e.g., one second) followed by a short pulse of high-power operation (e.g., 10 milliseconds (“ms”)), to provide a low average output power. The second phenomenon is commonly addressed by reducing generated EMI in various ways allowing a reduction in the size of the X-capacitor, which enables reduction of the bleeder resistor current. The first phenomenon above is not usually addressed.
Even when the controller is disabled, it still draws a small but significant amount of power. Furthermore, the bleeder resistor coupled in parallel with the X-capacitor draws continuous power regardless of load level. While the X-capacitor size can be reduced somewhat by good EMI design practices, all adapters require at least a small X-capacitor to meet EMI requirements, resulting in bleeder resistor losses at no load.
These two types of power losses, while relatively small, have now become substantial hindrances to lowering no-load losses as industry requirements become stricter each year. Thus, despite the development of numerous strategies to reduce power losses of power adapters, no strategy has emerged to provide substantial reduction of power dissipation while an adapter provides minimal or no power to a load. Accordingly, what is needed in the art is a design approach and related method for a power adapter that enable further reduction of power converter losses without compromising end-product performance, and that can be advantageously adapted to high-volume manufacturing techniques.